Detection Thresholds for Light Pulses Superimposed on Backgrounds with Temporally Modulated Luminance

Abstract

We investigated the effects of sinusoidal modulation of the luminance of a large (17 deg) spatially homogeneous field on detection thresholds for a brief (7.5 ms) light pulse 46 min arc or 17 deg in size. The average luminance of the background field was at a high photopic level (2800 Td) throughout the experiment. Temporal frequency f (0.4 – 200 Hz) and contrast C (0.05 – 0.8) of the background modulation were varied, together with the timing t of the test pulse in the modulation cycle. As a function of f, thresholds reach maximum around 15 – 20 Hz, and steeply decline at higher frequencies. At frequencies above flicker fusion, thresholds do not depend on t, and are equal to the detection threshold on a constant background with the time-averaged luminance, in accordance with Talbot's law. At frequencies below 15 Hz, threshold elevations above threshold at constant background depend on f as a power law, with a power exponent (0.8 – 1.0) that is close to 1. Also threshold elevation as a function of modulation contrast conforms to a power law, with a similar power exponent (0.8 – 1.0). The effects of t are complex, and depend on both the frequency and the contrast of the background modulation. Our results yield constraints for models of luminance adaptation and contrast masking. In particular, the results at high frequencies indicate that the first stage of such models should consist of a linear low-pass filtering of the input luminance. The results at low frequency suggest a temporal derivative operation of the resulting signal. We discuss to what extent existing models can explain the dynamics of the measurements.